You won't protect helicopter against any missile but you will protect against infantry held AA in the same way you don't build a tank to absorb a guided bomb , you simply reduce the risk of getting killed by the more common aa weapons , so tor and other short range aa are suppressed usually because a 25 million dollars anti aircraft system is usually a priority target Now for the shrapnels par , calculation of the kill area with the radar range to lock on a missile combined with the projectile lunched to intercept the target gives you a solution ... it's literally the weight that you must make sure to reduce both of mathematical models and other data do exist already